Why is output not what is expected (Streams)?

Question

Java

Just trying to figure out multithreading.
Tried to make a small example:

public class App {
    public static final int N = 10_000_000;
    public static int counter = 0;

    public static void inc() {
        counter++;
    }

    public static void main(String[] args) throws InterruptedException {
        Object obj = new Object();

        Thread t0 = new Thread(() -> {
            synchronized (obj) {
                for (int i = 0; i < N; i++) {
                    inc();
                }
            }
        });

        t0.start();

        Thread t1 = new Thread(() -> {
            synchronized (obj) {
                for (int i = 0; i < N; i++) {
                    inc();
                }
            }
        });

        t1.start();

//        t0.join();
//        t1.join();

        System.out.println(counter);
    }
}

Why without join(), the program does not run as expected? Made 2 threads. Inside, each has a synchronized block on the same object. I expected that it would work like this: Since they are synchronized on the same object, the threads will not interfere with each other. First, one will end, then another will start working. And as a result, the number 20_000_000 will be displayed. But now I don't know why it doesn't work the way I thought.
Explain how this program works. And what does the join method mean? I thought that it means that at first one will be completed then, then another will start working. Did I understand correctly? And why then does it not work as I described above, in my opinion, synchronization on one object is the same as waiting for the completion of another thread (in the example above).

Answer 1

t0 launched - it started the increment,
followed by t1,
and immediately printed, that's how many threads managed to count so many and output
if you remove the comment, the program waits for thread 0 to end, then starts thread 1, and then displays the result to the console